Abstract:
In this dissertation two categories of adaptive beamforming algorithms have been
studied. In first category the adaptive beamforming has been applied for null steering
whereas in second category it has been applied for direction of arrival mismatch
problem to avoid performance degradation of the beamformers. New algorithms have
been contributed in both categories.
In case of first category a specific structure has been proposed which provides
independent steering of all the available nulls present in the radiation pattern of an
array antenna. The idea is based on decoupling of the complex weights employed with
each antenna element to provide adaptively by controlling their values. This results in
a proposed specific structure. The proposed structure is further improved by
incorporating sidelobe suppression capability. Second Order Cone Programming has
been used to get the appropriate set of weights to be utilized in the proposed structure.
Similarly, the method for improving beam symmetry around the desired signal
direction is also incorporated. These additional features are included over the cost of
number of steerable nulls. A tailored Genetic Algorithm is proposed to compute the
weight vector required to incorporate the proposed structure for beam symmetry.
The second part of dissertation is meant for the second category of adaptive
beamformers applied for direction of arrival mismatch problem. Performance of these
beamformers degrades severely whenever there is a mismatch between the presumed
and actual direction of desired signal impinging on an antenna array. A Robust
Generalized Sidelobe Canceller has been proposed in this domain as remedial
measure to restore the performance. The major advantage of proposed algorithm is
that it provides improved results without broadening the main beam. This feature is an
added advantage in comparison with the previously existing techniques. For this
purpose the blocking matrix present in GSC has been modified without disturbing the
quiescent weight vector. This results in robustness against signal look direction error
without broadening the main beam. The simulation results confirm the improved
performance of the beamformer.
Another approach in this domain is based on diagonal loading of signal and data
covariance matrices, involved in subsequent computations. The amount of this
diagonal loading level is very critical which must not exceed a specific level to ensure
the positive definite behavior of signal covariance matrix. This is a standard
requirement for the convergence of existing general rank algorithms. Currently, there
exists no reliable criterion for deciding the amount of diagonal loading level. In this
context a new algorithm has been contributed to decide the amount of diagonal
loading. Proposed algorithm is iterative in nature and uses the beam symmetry around
the presumed signal direction to decide the level.